AC power systems, as is well-known in the art, provide electric power in the form of voltage and current that alternate between positive and negative polarities, typically following a sine wave with a predetermined amplitude and frequency. In the U.S., nominal amplitude is 120 volts RMS, at a frequency of 60 Hz. There are many types of electrical equipment, such as motors and relays that depend on quality power supply conditions to operate properly. More specifically, it is often critical that peak line voltage actually attain the expected nominal peak voltage to avoid, e.g., undesirable motor vibration or inadvertent relay switching, among other possible impacts of power line voltage fluctuations or transients.
When line voltage does not reach its nominal peak, it is said that a voltage sag (also known as “voltage dip”) has occurred. A voltage sag, as understood by those skilled in the art, is a brief reduction in the voltage on an AC power system, typically on the order of about half a cycle (0.08 second) to a few seconds. A voltage sag is contrasted with an “undervoltage” condition that might last for, e.g., more than a few seconds.
From the basic power formula, power=voltage×current, it is known that for a given amount of power, a sudden increase in the amount of current being used will cause a corresponding decrease in the voltage supplying that current. Thus, for example, when a new load comes on line in a power system, that new load will sink a predetermined amount of current, thereby causing, at least momentarily (assuming no additional power can be instantaneously delivered), a corresponding reduction in voltage. Usually, such momentary voltage reductions are small enough that the voltage actually remains within normal tolerances, as utilities are adept at closely monitoring and controlling their power delivery systems. However, on occasion, when there is an unexpectedly large increase in current demand, or when system impedance is high, the voltage can drop significantly.
Although voltage sags can result from faults that occur in distant parts of a power system, many voltage sag events actually originate from within one's own facility. Some common causes of voltage sags include starting a large load, such as a motor or resistive heater, loose or defective wiring, and faults or short circuits in other equipment within the facility. The voltage sag phenomenon has been known for some time and there have been several documented devices to counteract this undesirable condition.
For example, U.S. Pat. No. 5,329,222 to Gyugyi et al. describes an apparatus for dynamic voltage restoration that includes a capacitive storage element and a controller that provides a corrective error signal based upon a deviation between the utility supply voltage and a nominal ideal voltage. Energy is then inserted in series to compensate for the line transient.
In the same field, U.S. Pat. No. 6,118,676 to Divan et al. discloses a dynamic voltage sag correction system that includes capacitive storage elements and line voltage monitoring. When a voltage sag condition is detected, a static bypass switch is opened, and a regulator storage module is controlled to provide a near normal output voltage signal to output terminals.
Other prior art patents describe voltage sag monitoring and compensation both in the DC and AC realm.
Notwithstanding these known systems, there remains a need to provide practical and rapid reaction voltage sag compensation systems and methodologies.